It’s been a bad year for Eastern equine encephalitis in Massachusetts. The mosquito-borne virus claimed the life of a Fairhaven woman and infected six other people, including a 5-year-old Sudbury girl.

EEE rarely makes people ill, but when it does the consequences are grave: One-third of those infected die, and most of those who survive suffer permanent neurological damage.

The Globe recently asked four experts about whether EEE can be prevented, and why it is so bad in the first place. Here’s what we learned.

Why aren’t there any vaccines?

There are. Several, in fact. A vaccine is available for horses, and several other vaccines are in development. But none has been approved for use in humans.

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“The technology exists,” said Dan H. Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center. “It just hasn’t been made yet.”

Why not?

There’s no market for it because the virus is extremely rare.

“It’s not as much a scientific challenge as a financial challenge,” said Scott C. Weaver, scientific director of the Galveston National Laboratory at the University of Texas Medical Branch.

Weaver’s laboratory has tested a couple of promising vaccines in mice.

He was working with funding from the Department of Defense, but midway through, the agency changed course and funded research into a different virus. No pharmaceutical company has been interested in picking up the bill, because so few people would want the vaccine. “Typically, to bring a vaccine to licensure and market costs hundreds of millions of dollars,” Weaver said.

William B. Klimstra, a member of the Center for Vaccine Research at the University of Pittsburgh, has two different vaccines in development, one funded by the National Institutes of Health and one by the Department of Defense. His team has tested them in mice and expects to start testing nonhuman primates.

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Why can’t people take the horse vaccine?

The Army developed a human EEE vaccine similar to the horse vaccine, but it’s offered only to people who work in laboratories where they might be exposed.

It’s not an especially effective vaccine, in people or horses. At least two doses are required, and the immunity doesn’t last long. The vaccine has been approved for clinical trials in humans, Weaver said, so if a pharmaceutical company were interested, it could get started quickly.

Testing a vaccine against a rare illness is difficult, however.

The usual way is to vaccinate one group of people, inject a placebo in a comparable group, and see whether the vaccinated group stays healthy.

But because EEE is so rare, such a study wouldn’t provide meaningful results. Instead, researchers would have to demonstrate that the vaccine is effective in nonhuman primates by deliberately exposing them to the virus.

Why is the Pentagon interested in EEE?

Because the virus is a potential bio-weapon. EEE can be aerosolized and sprayed on troops. Inhaling the virus through the nose would give it direct access to the brain. It’s known that animals can be infected this way, and there have been a few scattered cases of people infected in the laboratory from inhaling the virus, Weaver said.

At first blush, EEE might not seem like a good candidate for a weapon, considering that many people bitten by EEE-infected mosquitoes don’t get sick. During a 1959 EEE outbreak in New Jersey, Weaver said, a study of blood samples found that for every person who fell ill with EEE, 20 other people had been infected but experienced no symptoms. But he said there have been few similar studies to replicate this.

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In any case, it’s likely that an aerosolized virus would be much more deadly. Spraying the virus would deliver a larger dose than a mosquito bite, and animal studies suggest sickness is more severe with aerosolized EEE.

Why does EEE cause severe illness?

EEE is dangerous because it bypasses the immune system’s first line of defense, the sentinel cells that alert the body that it has been infected, Klimstra said. The virus goes straight to the brain without hindrance or warning.

So people don’t get typical viral symptoms like headache, malaise, and fever. Instead, the first signs are neurologic: stiff neck, severe headache, fatigue.

“You don’t know you’re infected until the infection is very severe,” Klimstra said.

And there is no medication that attacks the virus and no specific treatment.

Why do EEE infections happen only in some years?

It’s not well understood. Massachusetts experienced EEE outbreaks in 2004-2006 and 2010-2012, with a few cases in between. The mosquitoes that carry EEE don’t typically bite people. But in some years, the virus is found in human-biting mosquitoes. It’s not clear what drives this change or why it happens every few years.

Where does that leave us?

With the usual advice to do what you can to prevent mosquito bites, such as by avoiding outdoor activities from dusk to dawn and using insect repellent. “It’s very rare,” said Tonya Colpitts, a Boston University microbiologist, “but it only takes one bite.”